The present invention relates to a process for separating a multi-component feed stream into fractions and apparatus for achieving the same. More specifically, the present invention relates to a process and apparatus which includes the use of a porous separator to separate a multi-component feed stream into fractions which contain relatively high concentrations of one or more of the components.
Numerous techniques are known for separating and recovering from a multi-component feed stream the individual components comprising the stream. Examples of such techniques include distillation, porous membrane separation, centrifugal separation, entrainment or impingement separation, and nonporous electrostatic membrane separation. Typically, these techniques are used separately for different types of applications and they are conducted in different types of equipment.
For example, U.S. Pat. No. 6,032,484 (Chernyakov), which is assigned to the same assignee as the present invention, describes a method for the separation and recovery of fluorochemicals from a gas stream containing a diluent gas and fluorochemicals by first contacting the gas stream with a system of membranes in one or more stages in which the membranes are selectively more permeable to the diluent gas than the fluorochemicals to provide a stream rich in the diluent gas and a stream rich in the gaseous fluorochemicals. Each resulting gaseous stream is purified subsequently in a separate step by use of distillation or adsorption to produce a stream highly enriched in the fluorochemicals and a stream highly enriched in the diluent gas.
The use of membrane separation and phase change techniques in combination with processes such as vacuum membrane distillation (VMD) and pervaporation are known also. VMD systems utilize a membrane-based process for extracting low concentrations of volatile organic compounds (VOCs) from a solvent such as water by partial vaporization through a polymeric membrane. The membrane acts as a selective barrier between the feed in the liquid phase and the vapor which permeates the membrane.
An example of a VMD process is described in the abstract of JP 3089922. According to this publication, a method is provided to separate a dissolved volatile substance from a feed stream by bringing the feed stream into contact with an inflow side of a porous separator, such as a hydrophobic porous membrane, and contacting an outflow side of the porous separator with an aqueous “recovery” solution. As the feed stream contacts the membrane, the component to be separated evaporates and forms a gaseous layer adjacent to the membrane. The component in gaseous form then enters the inflow side of the separator, diffuses though the separator, and exits the separator at the separator's outflow side. As the gaseous component exits the separator, it is absorbed into an aqueous recovery solution which is maintained at lower temperature than the feed stream. Thus, according to the process taught by this publication, the component to be separated does not undergo a phase change as it flows through the separator.
Another example of VMD is described in the abstract of JP 61018406. According to this publication, a feed liquid is brought into contact with a porous liquid-impermeable membrane and is then heated causing certain components of the feed to evaporate. The evaporated vapor permeates the membrane and enters a vapor-collecting space where it is condensed. Like JP 3089922, this publication teaches a process in which the component to be separated does not undergo a phase change as it flows through the separator.
In contrast to VMD processes, pervaporation involves the use of a membrane that functions as a selective barrier between the two phases, a liquid feed/retentate phase feed and a vapor phase permeate. The membrane allows the desired component(s) of the liquid feed to transfer through it by vaporization. This separation is mainly due to differences in polarity and not to the volatility difference of the components in the feed.
An example of a pervaporation process is found in U.S. Pat. No. 4,788,043 (Kagiyama) which describes a process for cleaning a semiconductor substrate with an organic solvent. As the substrate is cleaned, the organic solvent becomes contaminated with water, electrolytes and particulates. The solvent is purified by a two-step process involving a first pervaporization step, followed by a separate and independent distillation step. During the pervaporization step, the liquid solvent/water mixture is transferred through a pervaporator to remove a majority of water and other impurities from the solvent. More specifically, a majority of the solvent permeates a membrane of the pervaporator, while a majority of the water does not. The solvent that permeates through the membrane is then transferred to a separate device that removes an additional amount of water from the solvent via distillation. Thus, Kagiyama does not teach the use of a separator alone to effectively separate the components of a multi-component feed stream via permeation and creation of vapor and liquid permeant fractions.
A pervaporation process is also described in U.S. Pat. No. 4,900,402 (Kaschemekat) which discloses the separation of at least one component from a mixture of liquids, for example, separating ethanol from a fermentation mass by use of a first pervaporation device to form a first permeate vapor enriched in the component to be separated and (b) fractionating the first permeate vapor, for example, by temperature condensation, in a fractionating condenser to form a high concentration fraction twice enriched in the component to be separated. According to Kaschemekat, the mixture to be separated is subjected to a phase change (from liquid to vapor) as it permeates the membrane of the pervaporator. The vapor composition exiting the pervaporator is rich in ethanol. The enriched vapor is transferred to a condenser in which at least a portion of the water vapor is condensed into liquid and subsequently removed. While Kaschemekat teaches a process that utilizes a membrane to separate a multi-component feed stream via permeation and that the material permeating the membrane undergoes a phase change, it does not teach the use of a membrane alone to effectively separate the components of a multi-component feed stream via permeation and creation of vapor and liquid permeant fractions.
A multi-stage pervaporation process that utilizes multiple membranes in series and that is performed at progressively higher vacuum, higher temperature, or both, at each successive retentate stage is disclosed in U.S. Pat. No. 4,962,270 (Feimer). This process is described as being useful for separating components whose boiling temperatures vary over a wide range. Feimer is similar to Kaschemekat in that both teach a process for separating a multi-component feed via a pervaporator and a condenser. However, Feimer also teaches that multiple pervaporators and condensers may be used in series to form a plurality of process streams.
Other examples of pervaporation can be found, for example, in U.S. Pat. No. 5,108,549 (Wenzloff).
In each of the above-mentioned disclosures, the pervaporation or VMD step is not closely integrated with any other purification or separation processes. That is, the pervaporator only separates a multi-component feed stream via permeation. Thermodynamic separation (e.g., distillation) is taught as a discrete step which is performed in a separate piece of equipment.